MIDI is commands -- on/off/set-volume, basically. Converting audio into MIDI is a fairly large endeavor; it's not just a matter of simple wiring or a few components. If the instrument is digital internally, you might be able to gut it and rebuild it as a MIDI controller. If not...

Websearch says one of the higher-end models had MIDI out. If you're extremely lucky, the circuitry might basically be present and simply not brought out to a connector, or it might just need a few additional components. If not ... if it's basically a keyboard, you almost certainly could gut it and replace its current electronics with a microcontroller running a keyboard-to-MIDI encoder. Of course that involves murdering its current on-board synth. Maybe. Only one way to find out: either get your paws on a circuit diagram, or open one up and reverse-engineer it. Unless someone has already attempted this, which I couldn't tell from a very brief websearch.